1. Field of the Invention
This invention relates to a device for printing on paper or plate-shaped materials, such as plates made of glass, ceramic, glass-ceramic or plastic materials, having a transport device for the plates to be printed and an electrostatic, in particular an electrographic, printing device arranged above the transport device.
2. Description of Related Art
Such devices are known in various embodiments, as shown in European Patent References EP 0 834 784 A1, EP 0 727 778 A1 and EP 0 647 885 A1, and in U.S. Pat. No. 5,890,043. It is possible with these devices to apply toners containing ceramic inks to transfer media, which are used for decorating ceramic articles, such as plates, cups and the like.
With these devices the required print accuracy is only insufficiently achieved when directly printing on plate-like materials, such as glass, ceramic, glass-ceramic or plastic plates.
Screen-printing devices, which have a table-like conveying and centering unit as the transport device for the workpieces to be printed on, are also often employed for printing on such plate-shaped materials. An upper unit having a receptacle for a screen frame and a linear drive for a printing doctor blade is assigned to the base unit. Such screen-printing devices are sufficiently described in literature and their functioning is known. The conveying and centering unit in particular of such screen-printing devices today has a large degree of accuracy, repetitive accuracy and dependability. But the frequent screen changes in connection with changing print orders is disadvantageous, along with the odor from solvents, the solvent residue and the screen-printing residue.
It is one object of this invention to provide a device of the type mentioned above but in which a flexible printing process can be achieved in an efficient manner.
In accordance with this invention, this object is achieved with a conveying and centering unit of a screen-printing device which is combined as a transport device with the electrostatic, in particular the electrographic, printing device, which is compatible with the upper unit of the screen-printing device.
It is possible with this combination of a transport device of a screen-printing device with an electrostatic, in particular an electrographic, printing device, to fill changing print orders in an efficient manner even for smallest lots without having to accept the disadvantages of the screen-printing device. If the upper unit of the screen-printing device is compatible with the electrostatic, in particular the electrographic, printing device, the conveying and centering unit of a screen-printing device can be used for both devices for screen printing and electrostatic or electrographic printing methods.
In one embodiment, the electrostatic or electrographic printing device, the same as the upper unit of the screen-printing device, can be vertically lifted off the conveying and centering unit or tilted up from one end in order to simplify access to the conveying and centering unit. In this case the electrostatic or electrographic printing device, the same as the upper unit of a screen-printing device, can be arranged in a frame, which is connected with the conveying and centering unit and can be adjusted with respect to the latter.
In accordance with one embodiment, the structure of the electrostatic or electrographic printing device can be such that the electrostatic printing device has an endless belt, which is guided over two rollers and is tensed. An electrostatic pushbutton with an optical photoconductor roller and a developing unit is arranged above the upper run of the endless belt. On the side of the lower run of the endless belt facing away from the conveying and centering unit the toner can be transferred by a linearly guided electrostatic doctor blade unit from the endless belt to the workpiece to be printed.
For improving the linear toner transfer, an ultrasound unit is assigned to the electrostatic doctor blade unit. The release of the toner in particular is thus improved.
In accordance with a further embodiment, the electrostatic or electrographic printing device can also be varied in such a way that the electrostatic doctor blade device is embodied as a roller, which presses the endless belt from the side facing away from the workpiece to be printed on against the latter.
In a further embodiment of the electrostatic or electrographic printing device, the workpiece to be printed on is placed on a conductive plate and a prestress is applied to the plate and the electrostatic doctor blade device, which can be changed by a regulating device for adjusting the toner release. Thus it is possible to specifically match the toner release to the print distance, the type of toner, the plate thickness and the material of which the plate is made.
In a further embodiment, a roller-shaped transfer unit is integrated into the support frame of an upper unit of a screen-printing device, to which an electrostatic pushbutton with an optical photoconductor roller and developer unit is assigned. The circumferential speed of the roller of the transfer unit and its linear movement above the workpiece to be printed on are synchronized. It is possible to convert a screen-printing device to an electrostatic or electrographic printing method in a simple manner and cost-effectively.